Multiple satellite repeater capacity loading with multiple spread spectrum gateway antennas

ABSTRACT

A communication system (10), and a method executed by same, for allocating communications traffic through a plurality of satellites (12) of a constellation of low earth orbit satellites. Each of the plurality of satellites is oriented, at any given time when in view of a ground station (18), at a particular elevation angle. The method comprises the steps of: (a) providing each of the plurality of satellites with a receiver for receiving communication links from the ground station and a transmitter for transmitting communication links to user terminals; (b) in response to a request for service, determining if a highest elevation angle satellite can be assigned a new communications link; (c) if yes, assigning a new communication link to the highest elevation angle satellite; (d) if no, determining if a second highest elevation angle satellite can be assigned a new communications link; and (e) if yes, assigning a new communication link to the second highest elevation angle satellite. A number of different criteria can be employed in determining if a satellite can be assigned a new communication link, including: determining if the associated satellite has already been assigned a predetermined maximum number of communication links; and determining if the associated satellite, or a particular beam, is transmitting at or near to a power level that corresponds to a maximum peak flux density at the surface of the earth. Each of the steps of assigning is preferably accomplished such that the communication link is simultaneously established through at least two of the satellites to provide for diversity reception at a user&#39;s terminal (13).

CROSS-REFERENCE TO A RELATED PATENT APPLICATION

This patent application is a continuation of U.S. patent applicationSer. No. 08/465,972, filed Jun. 6, 1995, now U.S. Pat. No.: 5,592,481,issued on Jan. 7, 1997.

FIELD OF THE INVENTION

This invention relates in general to repeater-based communicationsystems.

BACKGROUND OF THE INVENTION

Satellite-based communications systems are well represented in the priorart. By example, reference is made to U.S. Pat. No. 5,303,286, whichissued on Apr. 12, 1994 to one of the inventors of this patentapplication, and which is entitled "Wireless Telephone/Satellite RoamingSystem". Reference is also made to the numerous U.S. Patents, foreignpatents, and other publications that are of record in U.S. Pat. No.5,303,286.

SUMMARY OF THE INVENTION

This invention is directed to a communication system, and to a methodexecuted by same, for allocating communications traffic through aplurality of satellites of a constellation of low earth orbitsatellites. Each of the plurality of satellites is oriented, at anygiven time when in view of a ground station, at a particular elevationangle. The method comprises the steps of (a) providing each of theplurality of satellites with a receiver for receiving communicationlinks from the ground station and a transmitter for transmittingreceived communication links to user terminals; (b) in response to arequest for service, determining if a highest elevation angle satellitecan be assigned a new communications link; (c) if yes, assigning a newcommunication link to the highest elevation angle satellite; (d) if no,determining if a second highest elevation angle satellite can beassigned a new communications link; and (e) if yes, assigning a newcommunication link to the second highest elevation angle satellite.Alternatively, a plurality of the satellites can be employed to transmitthe communication link to the user terminal.

A number of criteria can be employed in determining if a satellite canbe assigned a new communication link. For example, each of the steps ofdetermining may each include a step of determining if the associatedsatellite has already been assigned a predetermined maximum number ofcommunication links. Also by example, each of the steps of determiningmay each include a step of determining if the associated satellite istransmitting at or near to a power level that corresponds to a maximumpeak flux density at the surface of the earth. Further by example, itcan be determined if a given satellite is transmitting at or near thesatellite's peak transmit power. A further determination can be based onwhich satellite will be in view of both the user and the gateway forsome predetermined period of time.

In a presently preferred embodiment of this invention each satellitetransmits a plurality of beams to the earth. Thus, and in accordancewith a further embodiment of the method, each of the steps ofdetermining may each include a step of determining if a beam to whichthe new communication link would be assigned is operating at a maximumpredetermined power level, or is transmitting at or near to a powerlevel that corresponds to a maximum peak flux density.

The use of various combinations of the foregoing criteria can also beemployed when assigning satellites for new or handoff communications.

Further in accordance with a presently preferred embodiment of thisinvention each of the steps of assigning include the further steps of:(i) determining if the communication link is to be operated in adiversity reception mode at the corresponding user's terminal; (ii) ifyes, determining if at least one next lower elevation angle satellitecan be assigned a new communications link; and (iii) if yes, alsoassigning the communication link to the next lower elevation anglesatellite such that the communication link is simultaneously establishedthrough at least two of the satellites.

BRIEF DESCRIPTION OF THE DRAWINGS

The above set forth and other features of the invention are made moreapparent in the ensuing Detailed Description of the Invention when readin conjunction with the attached Drawings, wherein:

FIG. 1 is block diagram of a satellite communication system that isconstructed and operated in accordance with a presently preferredembodiment of this invention;

FIG. 2 is a block diagram of one of the gateways of FIG. 1;

FIG. 3A is a block diagram of the communications payload of one of thesatellites of FIG. 1;

FIG. 3B illustrates a portion of the beam pattern that is transmittedfrom one of the satellites of FIG. 1;

FIG. 4 is a block diagram that depicts the ground equipment support ofsatellite telemetry and control functions;

FIG. 5 is block diagram of the CDMA sub-system of FIG. 2;

FIG. 6 is a simplified system block diagram that is useful inillustrating the teaching of this invention; and

FIG. 7 is a logic flow diagram that illustrates a method of assigningcommunication traffic in accordance with this invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a presently preferred embodiment of a satellitecommunication system 10 that is suitable for use with the presentlypreferred embodiment of this invention. Before describing this inventionin detail, a description will first be made of the communication system10 so that a more complete understanding may be had of the presentinvention.

The communications system 10 may be conceptually sub-divided into aplurality of segments 1, 2, 3 and 4. Segment 1 is referred to herein asa space segment, segment 2 as a user segment, segment 3 as a ground(terrestrial) segment, and segment 4 as a telephone systeminfrastructure segment.

In the presently preferred embodiment of this invention there are atotal of 48 satellites in, by example, a 1414 km Low Earth Orbit (LEO).The satellites 12 are distributed in eight orbital planes with sixequally-spaced satellites per plane (Walker constellation). The orbitalplanes are inclined at 52 degrees with respect to the equator and eachsatellite completes an orbit once every 114 minutes. This approachprovides approximately full-earth coverage with, preferably, at leasttwo satellites in view at any given time from a particular user locationbetween about 70 degree south latitude and about 70 degree northlatitude. As such, a user is enabled to communicate to or from nearlyany point on the earth's surface within a gateway (GW) 18 coverage areato or from other points on the earth's surface (by way of the PSTN), viaone or more gateways 18 and one or more of the satellites 12, possiblyalso using a portion of the telephone infrastructure segment 4.

It is noted at this point that the foregoing and ensuing description ofthe system 10 represents but one suitable embodiment of a communicationsystem within which the teaching of this invention may find use. Thatis, the specific details of the communication system are not to be reador construed in a limiting sense upon the practice of this invention.

Continuing now with a description of the system 10, a soft transfer(handoff) process between satellites 12, and also between individualones of 16 spot beams transmitted by each satellite (FIG. 3B), providesunbroken communications via a spread spectrum (SS), code divisionmultiple access (CDMA) technique. The presently preferred SS-CDMAtechnique is similar to the TIA/EIA Interim Standard, "MobileStation-Base Station Compatibility Standard for Dual-Mode WidebandSpread Spectrum Cellular System" TIA/EIA/IS-95, July 1993, althoughother spread spectrum and CDMA techniques and protocols can be employed.

The low earth orbits permit low-powered fixed or mobile user terminals13 to communicate via the satellites 12, each of which functions, in apresently preferred embodiment of this invention, solely as a "bentpipe" repeater to receive a communications traffic signal (such asspeech and/or data) from a user terminal 13 or from a gateway 18,convert the received communications traffic signal to another frequencyband, and to then re-transmit the converted signal. That is, no on-boardsignal processing of a received communications traffic signal occurs,and the satellite 12 does not become aware of any intelligence that areceived or transmitted communications traffic signal may be conveying.

Furthermore, there need be no direct communication link or links betweenthe satellites 12. That is, each of the satellites 12 receives a signalonly from a transmitter located in the user segment 2 or from atransmitter located in the ground segment 3, and transmits a signal onlyto a receiver located in the user segment 2 or to a receiver located inthe ground segment 3.

The user segment 2 may include a plurality of types of user terminals 13that are adapted for communication with the satellites 12. The userterminals 13 include, by example, a plurality of different types offixed and mobile user terminals including, but not limited to, handheldmobile radio-telephones 14, vehicle mounted mobile radio-telephones 15,paging/messaging-type devices 16, and fixed radio-telephones 14a. Theuser terminals 13 are preferably provided with omnidirectional antennas13a for bidirectional communication via one or more of the satellites12.

It is noted that the fixed radio-telephones 14a may employ a directionalantenna. This is advantageous in that it enables a reduction ininterference with a consequent increase in the number of users that canbe simultaneously serviced with one or more of the satellites 12.

It is further noted that the user terminals 13 may be dual use devicesthat include circuitry for also communicating in a conventional mannerwith a terrestrial cellular system.

Referring also to FIG. 3A, the user terminals 13 may be capable ofoperating in a full duplex mode and communicate via, by example, L-bandRF links (uplink or return link 17b) and S-band RF links (downlink orforward link 17a) through return and forward satellite transponders 12aand 12b, respectively. The return L band RF links 17b may operate withina frequency range of 1.61 GHz to 1.625 GHz, a bandwidth of 16.5 MHz, andare modulated with packetized digital voice signals and/or data signalsin accordance with the preferred spread spectrum technique. The forwardS band RF links 17a may operate within a frequency range of 2.485 GHz to2.5 GHz, a bandwidth of 16.5 MHz. The forward RF links 17a are alsomodulated at a gateway 18 with packetized digital voice signals and/ordata signals in accordance with the spread spectrum technique.

The 16.5 MHz bandwidth of the forward link is partitioned into 13channels with up to, by example, 128 users being assigned per channel.The return link may have various bandwidths, and a given user terminal13 may or may not be assigned a different channel than the channelassigned on the forward link. However, when operating in the diversityreception mode on the return link (receiving from two or more satellites12), the user is assigned the same forward and return link RF channelfor each of the satellites.

The ground segment 3 includes at least one but generally a plurality ofthe gateways 18 that communicate with the satellites 12 via, by example,a full duplex C band RF link 19 (forward link 19a (to the satellite),return link 19b (from the satellite)) that operates within a range offrequencies generally above 3 GHz and preferably in the C-band. TheC-band RF links bi-directionally convey the communication feeder links,and also convey satellite commands to the satellites and telemetryinformation from the satellites. The forward feeder link 19a may operatein the band of 5 GHz to 5.25 GHz, while the return feeder link 19b mayoperate in the band of 6.875 GHz to 7.075 GHz.

The satellite feeder link antennas 12g and 12h are preferably widecoverage antennas that subtend a maximum earth coverage area as seenfrom the LEO satellite 12. In the presently preferred embodiment of thecommunication system 10 the angle subtended from a given LEO satellite12 (assuming 10° elevation angles from the earth's surface) isapproximately 110°. This yields a coverage zone that is approximately3600 miles in diameter.

The L-band and the S-band antennas are multiple beam antennas thatprovide coverage within an associated terrestrial service region. TheL-band and S-band antennas 12d and 12c, respectively, are preferablycongruent with one another, as depicted in FIG. 3B. That is, thetransmit and receive beams from the spacecraft cover the same area onthe earth's surface, although this feature is not critical to theoperation of the system 10.

As an example, several thousand full duplex communications may occurthrough a given one of the satellites 12. In accordance with a featureof the system 10, two or more satellites 12 may each convey the samecommunication between a given user terminal 13 and one of the gateways18. This mode of operation, as described in detail below, thus providesfor diversity combining at the respective receivers, leading to anincreased resistance to fading and facilitating the implementation of asoft handoff procedure.

It is pointed out that all of the frequencies, bandwidths and the likethat are described herein are representative of but one particularsystem. Other frequencies and bands of frequencies may be used with nochange in the principles being discussed. As but one example, the feederlinks between the gateways and the satellites may use frequencies in aband other than the C-band (approximately 3 GHz to approximately 7 GHz),for example the Ku band (approximately 10 GHz to approximately 15 GHz)or the Ka band (above approximately 15 GHz).

The gateways 18 function to couple the communications payload ortransponders 12a and 12b (FIG. 3A) of the satellites 12 to the telephoneinfrastructure segment 4. The transponders 12a and 12b include an L-bandreceive antenna 12c, S-band transmit antenna 12d, C-band power amplifier12e, C-band low noise amplifier 12f, C-band antennas 12g and 12h, L bandto C band frequency conversion section 12i, and C band to S bandfrequency conversion section 12j. The satellite 12 also includes amaster frequency generator 12k and command and telemetry equipment 121.

Reference in this regard may also be had to U.S. Pat. No. 5,422,647, byE. Hirshfield and C. A. Tsao, entitled "Mobile Communications SatellitePayload" (U.S. Ser. No. 08/060,207).

The telephone infrastructure segment 4 is comprised of existingtelephone systems and includes Public Land Mobile Network (PLMN)gateways 20, local telephone exchanges such as regional public telephonenetworks (RPTN) 22 or other local telephone service providers, domesticlong distance networks 24, international networks 26, private networks28 and other RPTNs 30. The communication system 10 operates to providebidirectional voice and/or data communication between the user segment 2and Public Switched Telephone Network (PSTN) telephones 32 and non-PSTNtelephones 32 of the telephone infrastructure segment 4, or other userterminals of various types, which may be private networks.

Also shown in FIG. 1 (and also in FIG. 4), as a portion of the groundsegment 3, is a Satellite Operations Control Center (SOCC) 36, and aGround Operations Control Center (GOCC) 38. A communication path, whichincludes a Ground Data Network (GDN) 39 (see FIG. 2), is provided forinterconnecting the gateways 18 and TCUs 18a, SOCC 36 and GOCC 38 of theground segment 3. This portion of the communications system 10 providesoverall system control functions.

FIG. 2 shows one of the gateways 18 in greater detail. Each gateway 18includes up to four dual polarization RF C-band sub-systems eachcomprising a dish antenna 40, antenna driver 42 and pedestal 42a, lownoise receivers 44, and high power amplifiers 46. All of thesecomponents may be located within a radome structure to provideenvironmental protection.

The gateway 18 further includes down converters 48 and up converters 50for processing the received and transmitted RF carrier signals,respectively. The down converters 48 and the up converters 50 areconnected to a CDMA sub-system 52 which, in turn, is coupled to thePublic Switched Telephone Network (PSTN) though a PSTN interface 54. Asan option, the PSTN could be bypassed by using satellite-to-satellitelinks.

The CDMA sub-system 52 includes a signal summer/switch unit 52a, aGateway Transceiver Subsystem (GTS) 52b, a GTS Controller 52c, a CDMAInterconnect Subsystem (CIS) 52d, and a Selector Bank Subsystem (SBS)52e. The CDMA sub-system 52 is controlled by a Base Station Manager(BSM) 52f and functions in a manner similar to a CDMA-compatible (forexample, an IS-95 compatible) base station. The CDMA sub-system 52 alsoincludes the required frequency synthesizer 52g and a Global PositioningSystem (GPS) receiver 52h.

The PSTN interface 54 includes a PSTN Service Switch Point (SSP) 54a, aCall Control Processor (CCP) 54b, a Visitor Location Register (VLR) 54c,and a protocol interface 54d to a Home Location Register (HLR). The HLRmay be located in the cellular gateway 20 (FIG. 1) or, optionally, inthe PSTN interface 54.

The gateway 18 is connected to telecommunication networks through astandard interface made through the SSP 54a. The gateway 18 provides aninterface, and connects to the PSTN via Primary Rate Interface (PRI).The gateway 18 is further capable of providing a direct connection to aMobile Switching Center (MSC).

The gateway 18 provides SS-7 ISDN fixed signalling to the CCP 54b. Onthe gateway-side of this interface, the CCP 54b interfaces with the CIS52d and hence to the CDMA sub-system 52. The CCP 54b provides protocoltranslation functions for the system Air Interface (AI), which may besimilar to the IS-95 Interim Standard for CDMA communications.

Blocks 54c and 54d generally provide an interface between the gateway 18and an external cellular telephone network that is compatible, forexample, with the IS-41 (North American Standard, AMPS) or the GSM(European Standard, MAP) cellular systems and, in particular, to thespecified methods for handling roamers, that is, users who place callsoutside of their home system. The gateway 18 supports user terminalauthentication for system 10/AMPS phones and for system 10/GSM phones.In service areas where there is no existing telecommunicationsinfrastructure, an HLR can be added to the gateway 18 and interfacedwith the SS-7 signalling interface.

A user making a call out of the user's normal service area (a roamer) isaccommodated by the system 10 if authorized. In that a roamer may befound in any environment, a user may employ the same terminal equipmentto make a call from anywhere in the world, and the necessary protocolconversions are made transparently by the gateway 18. The protocolinterface 54d is bypassed when not required to convert, by example, GSMto AMPS.

It is within the scope of the teaching of this invention to provide adedicated, universal interface to the cellular gateways 20, in additionto or in place of the conventional "A" interface specified for GSMmobile switching centers and vendor-proprietary interfaces to IS-41mobile switching centers. It is further within the scope of thisinvention to provide an interface directly to the PSTN, as indicated inFIG. 1 as the signal path designated PSTN-INT.

Overall gateway control is provided by the gateway controller 56 whichincludes an interface 56a to the above-mentioned Ground Data Network(GDN) 39 and an interface 56b to a Service Provider Control Center(SPCC) 60. The gateway controller 56 is generally interconnected to thegateway 18 through the BSM 52f and through RF controllers 43 associatedwith each of the antennas 40. The gateway controller 56 is furthercoupled to a database 62, such as a database of users, satelliteephemeris data, etc., and to an I/O unit 64 that enables servicepersonnel to gain access to the gateway controller 56. The GDN 39 isalso bidirectionally interfaced to a Telemetry and Command (T&C) unit 66(FIGS. 1 and 4).

Referring to FIG. 4, the function of the GOCC 38 is to plan and controlsatellite utilization by the gateways 18, and to coordinate thisutilization with the SOCC 36. In general, the GOCC 38 analyses trends,generates traffic plans, allocates satellite 12 and system resources(such as, but not limited to, power and channel allocations), monitorsthe performance of the overall system 10, and issues utilizationinstructions, via the GDN 39, to the gateways 18 in real time or inadvance.

The SOCC 36 operates to maintain and monitor orbits, to relay satelliteusage information to the gateway for input to the GOCC 38 via the GDN39, to monitor the overall functioning of each satellite 12, includingthe state of the satellite batteries, to set the gain for the RF signalpaths within the satellite 12, to ensure optimum satellite orientationwith respect to the surface of the earth, in addition to otherfunctions.

As described above, each gateway 18 functions to connect a given user tothe PSTN for both signalling, voice and/or data communications and alsoto generate data, via database 62 (FIG. 2), for billing purposes.Selected gateways 18 include a Telemetry and Command Unit (TCU) 18a forreceiving telemetry data that is transmitted by the satellites 12 overthe return link 19b and for transmitting commands up to the satellites12 via the forward link 19a. The GDN 39 operates to interconnect thegateways 18, GOCC 38 and the SOCC 36.

In general, each satellite 12 of the LEO constellation operates to relayinformation from the gateways 18 to the users (C band forward link 19ato S band forward link 17a), and to relay information from the users tothe gateways 18 (L band return link 17b to C band return link 19b). Thisinformation includes SS-CDMA synchronization and paging channels, inaddition to power control signals. Various CDMA pilot channels may alsobe used to monitor interference on the forward link. Satellite ephemerisupdate data is also communicated to each of the user terminals 13, fromthe gateway 18, via the satellites 12. The satellites 12 also functionto relay signalling information from the user terminals 13 to thegateway 18, including access requests, power change requests, andregistration requests. The satellites 12 also relay communicationsignals between the users and the gateways 18, and may apply security tomitigate unauthorized use.

In operation, the satellites 12 transmit spacecraft telemetry data thatincludes measurements of satellite operational status. The telemetrystream from the satellites, the commands from the SOCC 36, and thecommunications feeder links 19 all share the C band antennas 12g and12h. For those gateways 18 that include a TCU 18a the received satellitetelemetry data may be forwarded immediately to the SOCC 36, or thetelemetry data may be stored and subsequently forwarded to the SOCC 36at a later time, typically upon SOCC request. The telemetry data,whether transmitted immediately or stored and subsequently forwarded, issent over the GDN 39 as packet messages, each packet message containinga single minor telemetry frame. Should more than one SOCC 36 beproviding satellite support, the telemetry data is routed to all of theSOCCs.

The SOCC 36 has several interface functions with the GOCC 38. Oneinterface function is orbit position information, wherein the SOCC 36provides orbital information to the GOCC 38 such that each gateway 18can accurately track up to four satellites that may be in view of thegateway. This data includes data tables that are sufficient to allow thegateways 18 to develop their own satellite contact lists, using knownalgorithms. The SOCC 36 is not required to known the gateway trackingschedules. The TCU 18a searches the downlink telemetry band and uniquelyidentifies the satellite being tracked by each antenna prior to thepropagation of commands.

Another interface function is satellite status information that isreported from the SOCC 36 to the GOCC 38. The satellite statusinformation includes both satellite/transponder availability, batterystatus and orbital information and incorporates, in general, anysatellite-related limitations that would preclude the use of all or aportion of a satellite 12 for communications purposes.

An important aspect of the system 10 is the use of SS-CDMA inconjunction with diversity combining at the gateway receivers and at theuser terminal receivers. Diversity combining is employed to mitigate theeffects of fading as signals arrive at the user terminals 13 or thegateway 18 from multiple satellites over multiple and different pathlengths. Rake receivers in the user terminals 13 and the gateways 18 areemployed to receive and combine the signals from multiple sources. As anexample, a user terminal 13 or the gateway 18 provides diversitycombining for the forward link signals or the return link signals thatare simultaneously received from and transmitted through the multiplebeams of the satellites 12.

In this regard the disclosure of U.S. Pat. No. 5,233,626, issued Aug. 3,1993 to Stephen A. Ames and entitled "Repeater Diversity Spread SpectrumCommunication System", is incorporated by reference herein in itsentirety.

The performance in the continuous diversity reception mode is superiorto that of receiving one signal through one satellite repeater, andfurthermore there is no break in communications should one link be lostdue to shadowing or blockage from trees or other obstructions that havean adverse impact on the received signal.

The multiple, directional, antennas 40 of a given one of the gateways 18are capable of transmitting the forward link signal (gateway to userterminal) through different beams of one or more satellites 12 tosupport diversity combining in the user terminals 13. Theomnidirectional antennas 13a of the user terminals 13 transmit throughall satellite beams that can be "seen" from the user terminal 13.

Each gateway 18 supports a transmitter power control function to addressslow fades, and also supports block interleaving to address medium tofast fades. Power control is implemented on both the forward and reverselinks. The response time of the power control function is adjusted toaccommodate for a worst case 30 msec satellite round trip delay.

The block interleavers (53d, 53e, 53f, FIG. 5) operate over a blocklength that is related to vocoder 53g packet frames. An optimuminterleaver length trades off a longer length, and hence improved errorcorrection, at the expense of increasing the overall end-to-end delay. Apreferred maximum end-to-end delay is 150 msec or less. This delayincludes all delays including those due to the received signal alignmentperformed by the diversity combiners, vocoder 53g processing delays,block interleaver 53d-53f delays, and the delays of the Viterbi decoders(not shown) that form a portion of the CDMA sub-system 52.

FIG. 5 is a block diagram of the forward link modulation portion of theCDMA sub-system 52 of FIG. 2. An output of a summer block 53a feeds afrequency agile up-converter 53b which in turn feeds the summer andswitch block 52a. The telemetry and control (T&C) information is alsoinput to the block 52a.

An unmodulated direct sequence SS pilot channel generates an all zerosWalsh Code at a desired bit rate. This data stream is combined with ashort PN code that is used to separate signals from different gateways18 and different satellites 12. If used, the pilot channel is modulo 2added to the short code and is then QPSK or BPSK spread across the CDMAFD RF channel bandwidth. The following different pseudonoise (PN) codeoffsets are provided: (a) a PN code offset to allow a user terminal 13to uniquely identify a gateway 18; (b) a PN code offset to allow theuser terminal 13 to uniquely identify a satellite 12; and (c) a PN codeoffset to allow the user terminal 13 to uniquely identify a given one ofthe 16 beams that is transmitted from the satellite 12. Pilot PN codesfrom different ones of the satellites 12 are assigned differenttime/phase offsets from the same pilot seed PN code.

If used, each pilot channel that is transmitted by the gateway 18 may betransmitted at a higher or lower power level than the other signals. Apilot channel enables a user terminal 13 to acquire the timing of theforward CDMA channel, provides a phase reference for coherentdemodulation, and provides a mechanism to perform signal strengthcomparisons to determine when to initiate handoff. The use of the pilotchannel is not, however, mandatory, and other techniques can be employedfor this purpose.

The Sync channel generates a data stream that includes the followinginformation: (a) time of day; (b) transmitting gateway identification;(c) satellite ephemeris; and (d) assigned paging channel. The Sync datais applied to a convolution encoder 53h where the data isconvolutionally encoded and subsequently block interleaved to combatfast fades. The resulting data stream is modulo two added to thesynchronous Walsh code and QPSK or BPSK spread across the CDMA FD RFchannel bandwidth.

The Paging channel is applied to a convolutional encoder 53i where it isconvolutionally encoded and is then block interleaved. The resultingdata stream is combined with the output of a long code generator 53j.The long PN code is used to separate different user terminal 13 bands.The paging channel and the long code are modulo two added and providedto a symbol cover where the resulting signal is modulo two added to theWalsh Code. The result is then QPSK or BPSK spread across the CDMA FD RFchannel bandwidth.

In general, the paging channel conveys several message types whichinclude: (a) a system parameter message; (b) an access parametermessage; and (c) a CDMA channel list message.

The system parameter message includes the configuration of the pagingchannel, registration parameters, and parameters to aid in acquisition.The access parameters message includes the configuration of the accesschannel and the access channel data rate. The CDMA channel list messageconveys, if used, an associated pilot identification and Walsh codeassignment.

The vocoder 53k encodes the voice into a PCM forward traffic datastream. The forward traffic data stream is applied to a convolutionalencoder 531 where it is convolutionally encoded and then blockinterleaved in block 53f. The resulting data stream is combined with theoutput of a user long code block 53k. The user long code is employed toseparate different subscriber channels. The resulting data stream isthen power controlled in multiplexer (MUX) 53m, modulo two added to theWalsh code, and then QPSK or BPSK spread across the CDMA FD RFcommunication channel bandwidth.

The gateway 18 operates to demodulate the CDMA return link(s). There aretwo different codes for the return link: (a) the zero offset code; and(b) the long code. These are used by the two different types of returnlink CDMA Channels, namely the access channel and the return trafficchannel.

For the access channel the gateway 18 receives and decodes a burst onthe access channel that requests access. The access channel message isembodied in a long preamble followed by a relatively small amount ofdata. The preamble is the user terminal's long PN code. Each userterminal 13 has a unique long PN code generated by a unique time offsetinto the common PN generator polynomial.

After receiving the access request, the gateway 18 sends a message onthe forward link paging channel (blocks 53e, 53i, 53j) acknowledgingreceipt of the access request and assigning a Walsh code to the userterminal 13 to establish a traffic channel. The gateway 18 also assignsa frequency channel to the user terminal 13. Both the user terminal 13and the gateway 18 switch to the assigned channel element and beginduplex communications using the assigned Walsh (spreading) code(s).

The return traffic channel is generated in the user terminal 13 byconvolutionally encoding the digital data from the local data source orthe user terminal vocoder. The data is then block interleaved atpredetermined intervals and is applied to a 128-Ary modulator and a databurst randomizer to reduce clashing. The data is then added to the zerooffset PN code and transmitted through one or more of the satellites 12to the gateway 18.

The gateway 18 processes the return link by using, by example, a FastHadamard Transform (FHT) to demodulate the 128-Ary Walsh Code andprovide the demodulated information to the diversity combiner.

The foregoing has been a description of a presently preferred embodimentof the communication system 10. A description is now made of presentlypreferred embodiments of the present invention.

FIG. 6 is a simplified system block diagram that is useful inillustrating the teaching of this invention. System elements that havebeen described above are numbered accordingly in FIG. 6. Two gateways 18(designated 18 and 18') are illustrated as transmitting through threesatellites 12. The satellites 12 are designated as 12, having a highestelevation angle, satellite 12', having a second highest elevation angle,and satellite 12", having a lowest elevation angle.

A first forward link is established by frequency determining unit 43which transmits a single communication signal with a same frequencythrough transmitter antennas 40a, 40b and 40c of gateway 18 to thesatellites 12, 12' and 12". The satellites 12, 12' and 12" repeat thereceived signals and transmit them to the user terminal 13 with the samefrequency. Because the satellites 12, 12' and 12" are not co-located,each of the downlink transmissions will arrive at the user terminal 13at a different time and thus experience a different multipath delay. Thediversity combiner within the user terminal 13 combines the demodulatedreceived signals and delivers the intended signal to the user.

A second forward link is established by the frequency determining unit43' which transmits a single communication signal with the samefrequency through transmitter antennas 40a and 40b of a second and up toN gateways 18' to the satellites 12' and 12'. The satellites 12' and 12"repeat the received signals and transmit them to the user terminal 13'with the same frequency. As before, and because the satellites 12' and12" are not co-located, each of the downlink transmissions arrives atthe user terminal 13' with a different multipath delay. The diversitycombiner within the user terminal 13' combines the demodulated receivedsignals and delivers the intended signal to the user.

Each of the frequency determining units 43 and 43' sends, under controlof the associated controller 56 and 56', respectively, the uplinktransmissions in accordance with a method that is detailed below.

It should be realized that the relative elevation angles of thesatellites 12 change with respect to one another, as viewed from anyparticular gateway 18, as the satellites pass over the surface of theearth during each orbit. It should further be realized that the twogateways 18 and 18' may be separated from one another by hundreds orthousands of kilometers.

There may be a large number of users associated with each of thegateways 18. Furthermore, the constellation of satellites will have somenumber of satellites 12 that are simultaneously in view of the users andgateways. Generally the most advantageous communications operation hasthe users loaded onto the highest elevation angle satellite that is inview, in this case the satellite 12. However, to mitigate the effects ofblocking and shadowing it is desirable to transmit through as manysatellites as possible, even though some of the satellites that are inview may not be over the land mass where the user terminals 13 and 13'are located.

The transmitted spectrum is divided into several Frequency Division (FD)segments. Each segment is associated with a particular one of thegateways 18, and may or may not be present in all of the satellitebeams. Control of the loading of the satellites 12 is based on themethods described below which may be executed by the master controllerthat is preferably resident within the GOCC 38, and which is transmittedto the controller 56 of the gateways 18 over the GDN 58. The methods mayalso be executed by the gateways 18, in cooperation with informationreceived from the GOCC 38. The goal is to effect the allocation ofsystem resources so as to load the satellites 12 such that no onesatellite is overloaded with respect to the other satellites of theconstellation that are within view of a gateway or gateways 18. Oneembodiment of the method operates in real time or near real time, whileanother embodiment operates in a predictive mode.

It is first noted that the GOCC 38 is aware of the total transmittedpower of each satellite, in that the GOCC 38 has overall control of theloading of the satellites, and maintains a record of how many links areestablished between all of the gateways 18 and the satellites 12.Referring to FIG. 7, which assumes for this example that there are threesatellites (12, 12' and 12") presently in view of the gateways 18 and18', a request for service (Block A) is made to one of the gateways 18or 18'. The request for service may be made in response to a need toestablish a communication link with one of the user terminals 13, or maybe generated so as to handoff an already established link from onesatellite to another.

At Block B a determination is made if the highest elevation anglesatellite is fully loaded with communications traffic. If Yes, at BlockC a determination is made if the second highest elevation anglesatellite is fully loaded with communications traffic. If Yes, at BlockD a determination is made if the lowest elevation angle satellite isfully loaded with communications traffic. If Yes, at Block E the user isnot assigned to any satellite as all of the satellites in view of aparticular one of the gateways 18 are fully loaded.

If No at Block B the user is assigned to the highest elevation anglesatellite 12, thereby establishing one communication link through onesatellite. At Block G a determination is made if diversity reception isto be granted to the user, it being remembered that diversity receptionis desired to mitigate the effects of fading. If No, the singlecommunication link is maintained and the method terminates. If Yes atBlock G control passes to Block C to determine if the second highestelevation angle satellite is fully loaded.

If No, then at Block H the user is assigned to the second highestelevation angle satellite 12' (if entering from Block B), or the user isalso assigned to the second highest elevation angle satellite 12' (ifentering from Block G).

At Block I a determination is made if diversity reception is to grantedto the user, or if further diversity reception is to be granted if theuser is already assigned to at least one satellite. If No the methodterminates. If Yes at Block I then control passes to Block D todetermine if the lowest highest elevation angle satellite is fullyloaded.

If No, then the user is assigned to the lowest elevation angle satellite12" (if entering from Block C), or the user is also assigned to thelowest elevation angle satellite 12" (if entering from Block I).

The decision as to whether to operate a particular user communicationwith diversity reception (Blocks G and I) can be based on a number ofcriteria, such as an actual system loading or a predicted system loadingover some time period, such as the next n minutes. For example, duringpeak communication traffic periods it may be desirable to not operate inthe diversity mode, or to operate in a reduced diversity mode through,by example, only two satellites.

The decisions made at Blocks B, C, and D can also be based on a numberof different criteria. For example, one criterion may be based onwhether a satellite is currently repeating some predetermined maximumnumber of communications links. If yes, then the satellite is consideredto be fully loaded.

Another criterion may be based on a total power output of a givensatellite. Another, related criterion is whether a given one of thesatellite beams is operating at or near a power level that would exceeda predetermined peak flux density (PFD) as measured on the earth, suchas -154 dBW/m² /4 kHz. By example, a given satellite may be operating at75% of the maximum total communication capacity, but a particular beamwithin which a new user requires service (or to which a user may behanded off) is operating at a power level that is equal to or very nearto a level that would cause the RF energy from the beam to exceed thePFD limit on the earth. For this case the satellite may be considered tobe fully loaded, and the user is thus assigned to a next lower elevationangle satellite.

The GOCC 38 may predict, based on current traffic loading and ahistorical record of traffic loading, an expected communications trafficloading over an interval of time. Based on this prediction one or moreof the gateways 18 can be instructed to assign all new users to one ormore of the satellites for, by example, the next five minutes.

Although described in the context of a spread spectrum communicationsystem, it should be realized that the teaching of this invention alsohas applicability to other satellite communication systems that mayutilize, by example, Time Division Multiple Access (TDMA) techniques.The teaching of this invention may also be applied to other than lowearth orbit satellite communication systems.

Thus, while the invention has been particularly shown and described withrespect to presently preferred embodiments thereof, it will beunderstood by those skilled in the art that changes in form and detailsmay be made therein without departing from the scope and spirit of theinvention.

What is claimed is:
 1. A method for allocating communications trafficamongst a plurality of satellites of a constellation of low earth orbitsatellites, each of the plurality of satellites being oriented, at anygiven time when in view of a user terminal, at a particular elevationangle, comprising the steps of:providing each of the plurality ofsatellites with a receiver for receiving communication links from aground station, transmitters for transmitting communication links touser terminals through frequency channels, receivers for receivingcommunication links from user terminals through the frequency channels,and a transmitter for transmitting communication links to the groundstation; in response to a request to one of assign a communication linkto a user terminal or to handoff an already assigned communication link,determining if a highest elevation angle satellite can be assigned a newcommunications link; if yes, assigning a new communication link to thehighest elevation angle satellite; if no, determining if a lowerelevation angle satellite can be assigned a new communications link; andif yes, assigning a new communication link to a lower elevation anglesatellite; wherein the steps of determining each include a step ofdetermining if the satellite will be in view of both the user terminaland the ground station for a predetermined period of time and, if not,the satellite is not accepted for being assigned the new communicationlink; and wherein the steps of determining are each also performed inaccordance with a predicted communications traffic loading for thesatellite.
 2. A method as set forth in claim 1, wherein each of thesteps of determining include the further steps of:determining if thecommunication link is to be operated in a diversity mode; if yes,determining if at least one next lower elevation angle satellite can beassigned a new communications link; and if yes, also assigning thecommunication link to the next lower elevation angle satellite such thatthe communication link is simultaneously conducted through at least twoof the satellites.
 3. A method for allocating communications trafficamongst a plurality of satellites of a constellation ofnon-geosynchronous satellites, each of the plurality of satellites beingoriented, at any given time when in view of a user terminal, at aparticular elevation angle, comprising the steps of:generating a requestto assign a communication link to a user terminal; in response to thegenerated request, operating a ground station to assign thecommunication link to at least two satellites that are selected ashaving the highest elevation angles relative to the user terminal, inaccordance with at least a current satellite communications trafficloading and a predicted satellite communications traffic loading foreach of the plurality of satellites that are in view of the userterminal and the ground station, so as not to exceed a maximum satellitecommunications traffic loading for a given one of the satellites; andoperating the user terminal so as to coherently combine a communicationsignal received from said at least two satellites over the assignedcommunication link.